1. Field of the Invention
This invention relates to an image signal processing system and, in particular, to a system for converting an analog image signal obtained by scanning a two-dimensional image into a binary image signal with the use of an adjustable threshold level.
2. Description of the Prior Art
In an imaging system of a facsimile and the like, if an image signal having waveform A shown in the time chart of FIG. 1 is used to produce an image and then it is processed through photo-electric conversion, the resultant image signal will have waveform B also shown in FIG. 1. As shown, the recovered image signal B is significantly corrupted as compared with the original image signal A. Such corruption in waveshape can be critical in the process of analog-to-digital image signal conversion. For example, if the corrupted image signal B is subjected to binary value conversion processing with the use of threshold level C, the narrow line section indicated by "PEAK" will fade away, i.e. the area to be recorded in black will be recovered in white, and, moveover, the narrow gap section between lines indicated by "VALLEY" will disappear, i.e., the area to be recorded in white will be recorded in black. This is disadvantageous because the recorded image will be blurred and some delicate information will be lost.
The prior art approach to cope with such a problem is to carry out the step of emphasizing high frequency components of an analog image signal prior to the step of threshold processing. In accordance with such a prior art technique, the corrupted signal B can be compensated as shown by waveform D, so that the information as to narrow lines and narrow gaps between lines may be preserved even if threshold processing is carried out. However, the prior art technique is ineffective in the case where narrow lines and/or narrow gaps between lines run in parallel with the scanning direction, and, moreover, it has a disadvantage of susceptibility to noise.